Journal of Magnetism and Magnetic Materials 76 & 77 (1988) 265-266 265 North-Holland, Amsterdam MAGNETORESISTANCE OF (Ce,La)Ru2Si z ALLOYS R. DJERBI, P. HAEN, F. LAPIERRE and J.-M. MIGNOT CRTBT*-CNRS, BP166X, F-38042 Grenoble, France Magnetoresistance measurements on Ce 1 xLaxRu2Si2 single crystals are reported. For x = 0.05 a maximum of p is observed at the metamagnetic-like transition field as in CeRuzSi 2. In the AF-ordered alloys, o(H) is always negative above Ts, while below TN it drops at the magnetic transitions and reproduces their hysteresis effects. The tetragonal heavy-fermion compound Ce- Ru 28i2 presents a sharp magnetoresistance maxi- mum when H is applied along the c-axis. Below 10 K, this anomaly occurs at exactly the same critical field HM 8.9 T at 10 K, 7.9 T at 1.3 K where an inflexion point (and even a step-like jump at 1.3 K) is observed in the M vs. H curves [1]. At higher temperatures, the positive curvature in M(HIIc ) disappears, but the maximum in p(H), although considerably broadened and shifted to lower magnetic fields, still can be de- tected up to 60 K. The latter temperature corre- sponds to the vanishing of the antiferromagnetic (AF) correlations observed in neutron-scattering experiments [2]. Substitution of Ce by La initially reduces H M, then leads to AF ordering for x > 0.08 in the Ce 1 xLaxRu2Si2 system [3]. Moreover, magnetization experiments [4] show the existence of a complex (H,T) phase diagram below Ty in the ordered alloys, and the persistence of the metamagnetic-like transition in the paramagnetic state for x < 0.13. This prompted us to perform magnetoresistance experiments in order to check how the effects observed in the pure compound are modified by La alloying. In this paper, we report data obtained on the same Ce 1 xLaxRu2Si2 single crystals as in ref. [5], with the current in the basal plane and HIP c. The measurements were performed down to = 1.7 K in a superconducting coil. Fig. 1 shows data for x = 0.05. Maxima in p(H) occur for this La con- centration as for pure CeRuzSi 2. However, these maxima are located at slightly lower fields than the values of H M derived from magnetization experiments [4] (for the two temperatures: 4.2 and 1.4 K where the latter were performed). This discrepancy between H(Pmax) and Hr,, contrary to what was observed for the pure compound, can ~ 20.5 20 11. ~ ~~ 10.! lg.. 7 5 i 1 I ~ I , , ) 2 4 6 8 H(T) Fig. l. Magnetoresistivity of Ceo.9~La0.osRu2Si2. Note the change of scale for each curve. For the two lowest tempera- tures, the arrows indicate the metamagnetic field value de- duced from magnetization measurements [4]. * Laboratoire associ6/~ l'Universit6 Joseph Fourier, Grenoble. is +-r--, \ 14- "Ha 113 < T:1.B6K H~,,." - -....... 6.5 0 H( ) 22 18 1/, 10 Fig. 2. Magnetoresistivity of Ceo.sTLao.13Ru2Si2. For T=1.86 K, the two transition fields are indicated by vertical arrows. 0304-8853/88/$03.50 © Elsevier Science Publishers B.V.